A known system for ejecting ink to form images on a moving web of media material is shown in FIG. 4. The system 10 includes a web unwinding unit 14, a printing apparatus 18, and a cutting station 22. In brief, the web unwinding unit 14 includes an actuator, such as an electrical motor, that rotates a roll of media material in a direction that removes a web 26 of media material from the unwinding unit 14. The web 26 is fed through the printing apparatus 18 along a path, which extends to the cutting station 22. The printer, referred to as a printing apparatus 18, treats the web 26 to remove debris and loose particulate matter from the web surface, ejects ink with numerous marking stations onto the moving web to form printed images, and then fixes the printed image to the web. The marking stations may eject different colored inks onto the web 26 to form a composite colored image. In one system 10, the marking stations eject cyan, magenta, yellow, and black ink for forming composite colored images. The web 26 is then pulled into the cutting station 22, which cuts the web into sheets for further processing.
The printing apparatus 18 uses a registration control method to control the timing of the ink ejections onto the web 26 as the web passes the marking stations. One known registration control method that may be used to operate the marking stations in the printing apparatus 18 is the single reflex method. In the single reflex method, the rotation of a single roller at or near a marking station is monitored by an encoder. The encoder may be a mechanical or electronic device that measures the angular velocity of the roller and generates a signal corresponding to the angular velocity of the roller. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller may adjust the linear web velocity calculation by using tension measurement signals generated by one or more loadcells that measure the tension on the web 26 near the roller. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another known registration control method that may be used to operate the marking stations in the printing apparatus 18 is the double reflex method. In the double reflex method, two rollers are each monitored by an encoder. One roller lies on the web path before the marking stations and the other roller lies on the web path following the marking stations. The angular velocity signals generated by the encoders for the two rollers are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web 26 at each roller and then to interpolate the linear velocity of the web at each of the marking stations. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing apparatus 18.
To address demand for printing systems 10 that use a large number of colored inks, some systems 10 include printing apparatus 18, such as the one shown in FIG. 4, arranged in tandem. The tandem arrangement enables the marking stations in the two printing apparatus 18 to use different colored inks. Additionally, a web inverter may be positioned between the two printing apparatus 18 to enable the web to be turned over so the reverse surface of the web may be printed by the second printing system. This printing system 10 configuration enables the entire width of the reverse side of the web to be printed.
One issue encountered in printing systems 10 having tandem connected printing apparatus 18 is the need to synchronize the registration between the first printing apparatus and the second printing apparatus. If the two printing apparatus 18 form images on the same side of the web, then slight differences in the printed images may adversely impact image quality. Even when the two printing apparatus 18 form images on different sides of the web, registration synchronization is still important because the duplex printed web is cut into individual, double sided printed pages. If the registration is not aligned well, an image on one side of the web may creep over the length of a print job into the cutting zone between images. Addressing the registration of the images printed by the two printing apparatus on a single web would be useful.